Ingredient file // Plant protein
Soy Protein Isolate
Real, digestible protein with an allergy caveat
Soy protein isolate is protein separated from defatted soybeans after much of their carbohydrate and fiber is removed. It raises a food's protein content, and dogs digest it well in short feeding trials. Dogs with a proven soy allergy should avoid it.
- What it is
- A dry soy protein powder made by separating protein from most of the bean's oil and carbohydrates.
- Made from
- Dehulled, defatted soybeans
- Main job
- Concentrated plant protein
- Typical amount
- Trials fed 18% in kibble
- Watch for
- A diagnosed soy allergy
- Not the same as
- Hydrolyzed soy protein
Our read // In our catalog
In 1 of every 519 foods we track
Share of foods that list it
0%
5 of the 2,595 foods with an ingredient list on file, from 2 brands. All dry dog food.
Each square is 1% of the catalog
Where it sits on the label
Median position 5. In the top five of 3 foods, so it is usually a main ingredient.
Above the salt
Almost always listed before the salt, so there is more of it than a pinch.
Price per lb
Median of the cheapest bag price across 3 priced foods, 75% above the catalog. The price is the food's, not the ingredient's.
Who leans on it
60%
of the foods that list it are from Royal Canin. 2 brands use it in total.
What it is
How it is made
Soy processors first remove the beans' hulls and oil. Isolate makers take the remaining flakes and separate protein from much of the carbohydrate, producing a concentrated powder for food and feed.4
- 01 // Source Soybean flakes Processors remove the hulls and oil, leaving flakes that still contain protein and carbohydrates.
- 02 // Process Protein extracted Water adjusted to a mildly alkaline pH dissolves much of the protein from the flakes.
- 03 // Process Protein separated The liquid is separated from solids, then acidified so the dissolved protein collects as a curd.
- 04 // Output Dried isolate The curd is washed and dried into a fine powder for the kibble plant.
What is in it
Composition, dry matter
90
% protein
The typical protein share reported by FAO. This is a protein ingredient, with little fat or carbohydrate left from the bean.
Why it is in the bag
What the manufacturer gets
-
01
Raises recipe protein
The powder contributes mostly protein by dry weight. It lets a manufacturer raise the crude protein figure without adding another animal ingredient.4,5
-
02
Limits soybean fiber
Separating the protein removes much of the bean's carbohydrate and fiber. In a short dog trial, the isolate diet produced less feces than the soybean meal diets.1,4
-
03
Targets a formula
Its concentrated protein gives formulators a predictable ingredient, but the extra separation costs money. FAO reports isolate costs substantially more than defatted soy flour.4
How much A controlled trial fed kibble containing 18% soy protein isolate to adult dogs for short study periods.1
Our read // The claims
What the evidence supports
Strong: controlled studies in dogs agree. Some: small or mixed dog data. Thin: mechanism or lab work only. None: nothing, or the evidence points the other way.
-
Critics say
It raises protein without adding meat
Evidence
- Strong
- Some
- Thin
- None
FAO lists a typical isolate at 90% protein on a dry basis. Its protein counts toward the food's crude protein figure even though it comes from soy. The figure alone does not tell you how much came from meat.4,5
-
Brands say
Dogs digest soy isolate well
Evidence
- Strong
- Some
- Thin
- None
In a 12-dog crossover trial, a diet containing 18% isolate had higher protein digestibility and less fecal output than the soybean meal diets. The formulas also differed in rice and fiber, so the result does not measure isolate alone.1
-
Brands say
Soy isolate treats liver disease
Outruns the evidence
Evidence
- Strong
- Some
- Thin
- None
In a 16-dog crossover study of congenital liver shunts, the soy diet lowered blood ammonia more than a poultry diet after 4 weeks. Clinical liver encephalopathy scores improved with both diets, without a significant difference between them.2
-
Owners say
Soy commonly causes food allergy in dogs
Outruns the evidence
Evidence
- Strong
- Some
- Thin
- None
A review found soy reactions in 18 of 297 dogs already identified as having food reactions, fewer than for beef, dairy or chicken. Those selected cases do not show that soy commonly causes allergy in dogs generally.3
-
Critics say
Soy isolate is cheap filler
Outruns the evidence
Evidence
- Strong
- Some
- Thin
- None
A typical isolate is 90% protein on a dry basis, and dogs ate an 18% isolate diet in a controlled trial. FAO reports that making isolate costs more than making defatted soy flour. It has a clear nutritional job.1,4
Straight answers
What owners ask
-
Q01
Is it just filler?
NoIt supplies concentrated protein. A bag's crude protein figure includes that plant protein, so read the surrounding ingredients to see what else supplies protein rather than dismissing the isolate as empty bulk.4,5
-
Q02
Is this hydrolyzed soy?
NoIsolating separates soy protein from other bean components. Hydrolyzing breaks protein into smaller pieces through an additional process. A label that says soy protein isolate alone does not describe a hydrolyzed allergy diet.4
-
Q03
Should a soy allergy worry me?
Only if diagnosedSoy was a trigger in 18 of 297 dogs with documented food reactions in one review. A dog with a proven soy reaction should avoid intact soy isolate; the ingredient is not a reason to expect allergy in every dog.3
Who uses it
Brands, best sellers and the company it keeps
Top brands, by foods that list it
Usually listed with, share of the foods that list it
- Brewers Rice 60%
- Chicken Fat 60%
- Natural Flavor 60%
- Soy Flour 40%
- Chicken 40%
- Chicken Meal 40%
- Fish Oil 40%
- Potassium Chloride 40%
- Corn 40%
- Dried Plain Beet Pulp 40%
- Beef By-Product 20%
- Soy Grits 20%
On the shelf
Foods that list it
5 foods in our reviews list it. Here are some of them.
Our read // On the bag
Count it as protein when reading the bag
If soy protein isolate sits high on the list, it contributes meaningfully to the protein figure. Look at the animal proteins around it and the food's nutritional adequacy statement before judging the recipe. Skip it for a dog with a proven soy allergy; otherwise, its presence alone is no reason to reject the food.
Sources
What we read
Checked October 1, 2026.
- 1 Digestibility and palatability of extruded dog food with Aspergillus oryzae fermented soybean meal and its influence on colonic fermentation Indicators
- 2 Soy Protein Isolate versus Meat-Based Low-Protein Diet for Dogs with Congenital Portosystemic Shunts
- 3 Critically appraised topic on adverse food reactions of companion animals (2): common food allergen sources in dogs and cats
- 4 Technology of production of edible flours and protein products from soybeans. Chapter 6.
- 5 What’s in the Ingredients List?